1. Field of the Invention
The present invention is related to semi-conductor devices. In particular, the present invention is related to buried fuse reading circuits.
2. Description of Related Art
In prior art fuse reading circuits, a blown buried (e.g., metal) fuse may be distinguished from an intact fuse by configuring a corresponding sense amplifier to change state when the fuse resistance exceeds a specified value. The fuse resistance may be determined by the sense amplifier as a function of a current, which passes through the sense amplifier and the fuse. Thus a threshold value for the current may be established corresponding to the specified fuse resistance to establish a trip point. The trip point corresponds to the value of fuse resistance, which causes the sense amplifier to change state.
The resistance of an intact fuse is typically on the order of a single digit and is well controlled. The resistance of a blown fuse, however, may depend on variables such as moisture and passivation properties and are thus more difficult to control or predict. A low trip point provides better margin to resist the effects of such variables at the expense of much higher currents. In fact, the current required by the sense amplifier is inversely proportional to the square of the trip point. To conserve energy, the sense amplifiers are turned off after the fuses have been read and their values latched. In battery powered devices or in large arrays of buried fuses, the total energy consumed by the fuse read process, which may be measured as I*V*t, becomes an important system consideration.
Variations in factors such as fabrication processing, ambient and internal temperatures, and power supply conspire to spread the value of currents and the settling time of the sense amplifier over a wide range. Traditional designs call for a timer to power up the sense amplifiers for a period corresponding to the maximum settling time required by the worst case values. Since the timer period is fixed and corresponds to worst case values, the energy consumption under best-case settling can be twice as much as necessary. Furthermore, a relatively accurate clocking mechanism must be incorporated into the device design having the undesirable effect of raising overall device costs.
Previous attempts to solve problems and disadvantages associated with prior art designs include: 1) using a clock and counter to measure out the time period required to read the fuses for worst-case conditions, which period is typically much longer than that needed for best-case and nominal conditions leading to extra energy consumed; 2) using lower currents but larger sense amplifier transistors; and 3) moving the trip point higher. Problems arise, however, in prior art solutions in that, for example, with regard to 1), a relatively accurate clock is needed and gives rise to additional energy to account for clock frequency variations and the need for additional silicon area; 2) the sense amplifier gain for larger transistors is inversely proportional to the square of the trip point and additional silicon area is needed; and 3) higher trip points lead to trade offs with regard to device robustness.